1. Field of the Invention
The invention relates to a press felt for use in a press section of a paper, board or tissue machine.
2. Description of the Related Art
Press felts known in the art comprise a base structure which provides the dimensional stability of the felt and a batt fiber structure which is normally attached to the base structure by needling and which provides a paper and a machine contacting side of the felt.
During operation especially, the batt fiber structure is subjected to compaction-expansion cycles when the felt runs through the press nip. During compaction, the felt provides a reduced water take-up capability. After leaving the press nip the felt has to recover as quickly as possible to avoid rewetting of the paper sheet.
After several hundred thousand of compaction-expansion cycles the degree of recovery of the felt to its original thickness reduces irreversibly, leading to reduced void volume and therefore reduced water take-up capability of the felt. When the water take-up capability drops below a certain level, the felt has to be removed from the machine.
In the past, several attempts have been made to enhance the life time of such press felts. Most attempts were directed to optimize the batt fiber structure by providing coarser fibers in the batt fiber structure or by filling the batt fiber structure with polymeric material, e.g., polymeric material in particulate form, to maintain the degree of recovery after compaction over longer time. From U.S. Pat. No. 7,871,672 it is, for example, known to partially fill the void volume of the batt fiber structure with a solution with polymeric material in particulate form and melting the polymeric material to generate a composite structure of polymeric material and batt fibers.
The above described attempts were only in some cases successful and relatively expensive in manufacturing.
What is needed is a press felt which has improved water take-up capability. What is further needed is a felt with enhanced durability to withstand more compaction-expansion cycles than felts known in the art and which is cheaper to manufacture than the above mentioned felts known in the art.